Zoom lens system for microfilm projectors



350-427 5;; RQQM 1957 D. E. GUSTAFSON 3,360,325

ZOOM LENS SYSTEM FOR MICROFILM PROJECTORS Filed May 27, 1964 l 'INVENTOR.

Darryl E gustafson.

United States Patent 3,360,325 ZOOM LENS SYSTEM FOR MICROFILM PROJECTORS Darryl E. Gustafson, Lincolnwood, Ill., assignor to Bell & Howell Company, Chicago, 11]., a corporation of Illinois Filed May 27, 1964, Ser. No. 370,517 1 Claim. (Cl. 350-184) ABSTRACT OF THE DISCLOSURE A mechanical compensation zoom lens system for use at finite conjugates made up of a positive zooming member in axial alignment with and between a fixed rear positive member and a front negative compensating member, in which the zooming member consists of a biconvex triplet component in front of a single positive element convex to the front and the compensating member consists of a negative triplet component concave toward the rear and in front of a single biconcave element.

This invention relates to zoom lens systems of the mechanical compensation type and to optical systems for projecting images at finite conjugates, such as enl-argers, table top projectors and particularly microfilm projectors.

The object of the invention is to provide an optical zoom system that is highly corrected at finite conjugates, particularly at magnification in the range from about 20X to about 40X.

A further object of the invention is to provide an optical zoom system that is highly corrected for use at magnifications in the range from about 20X to about 40X and operating between object and image planes that are a fixed distance apart.

A still further object of the invention is to provide an optical zoom system adapted to include an image-rotation (or anti-rotation) prism.

A particular object of the invention is to provide a zoom system in which the aberrations are well balanced for use in projecting different sizes of film area onto a constant size screen.

Zoom lens systems are well known, and comprise two or more members of which at least two are movable. In the so-called optical compensation type, the movements of the movable members are linearly related (usually equal) and the position of the image is maintained only approximately. In the mechanical compensation type, to which the present invention relates, there are two movable members controlled by cam action or some mechanical equivalent thereof. One movable member, called the zooming member has a greater effect on the magnification and a smaller effect on the image position than the other movable member, called the compensating member, and the image is maintained in exactly the same position during zooming within the degree of accuracy of the cam.

The front of the system is the end facing the longer conjugate in accordance with the usual convention.

According to the present invention, a mechanical compensation zoom lens system for use at finite conjugates is made up comprising a positive zooming member in axial alignment with and between a fixed rear positive member and a front negative compensating member, in which the zooming member consists of a biconvex triplet component in front of a single positive element convex to a front and the compensating member consists of a negative triplet component concave toward the rear and in front of a single biconcave element. The fixed rear positive member may be broadly of conventional construction. Ordinarily such a prime lens is designed by known procedures to'have aberrations equal and opposite to the average aberrations of the zooming portion of the system.

3,360,325 Patented Dec. 26, 1967 This ordinary approach is only partially efiective, however, in the particular case in which the screen size is constant, because there is no part of the lens system which will afiect the focal plane aberrations equally at all focal length positions. The only aberrations which may be treated in the usual manner are those which depend on aperture only and not on the field angle, namely spherical aberration and axial color. The remaining aberrations can be stabilized to a small degree in the variable focus section, but the major aberration balancing must be performed by treating the system in its entirety.

The computing of the lens positions for various degrees of zooming is a slightly longer process than in ordinary zoom systems because the distance from the screen to the compensating member enters as an additional variable. One convenient method of computing them comprises four stages as follows: The first stage is to trace rays from the short conjugate or film plane through the fixed mem-' ber and the zooming member for a series of positions of the zooming member to determine the several positions of the intermediate image plane. The second stage is to trace rays from the screen plane through the compensating member (the front member) for a series of screen-to-lens distances to determine the position of the image of the screen for each. The third stage is to interpolate from the latter series to match the intermediate image plane positions of the former series and optionally to compute through the whole system to verify the results. This is all that is needed for computing lens positions during design. The fourth stage, needed for designing the cam mechanism, is to interpolate as many additional values as required between the series of values computed in the first three stages.

It will be clear to those skilled in the art that if the zooming member passes through its unit-magnification position in the range chosen, then the compensating member moves first away from the screen and then back toward the screen when the system is zoomed from one end of its range to the other. However, in the particular example described below, this is not the case. The range chosen is such that when the front member is in its farthest forward position, the second member is in its farthest rearward position, and vice versa. This is done to provide more space for the image-rotating prism in accordance with a particular object of the invention.

According to a special form of the invention, the rear fixed positive member comprises a front negative portion or collimating lens, a rear positive portion or prime lens and an image rotating prism therebetween. Preferably, the collimating lens consists of a negative compound component and the prime lens consists of a plurality of positive components and a biconcave negative component between two of them. The light between the two portions is substantially collimated to avoid the introduction of aberrations as it passes through the rotation prism.

Of the several known types of rotation prisms, the

dove prism is preferred. A short prism (optically) is desirable to avoid vignetting the light near the edge of the field, and a clove prism made of high index glass is among the shortest known. Also, a direct-vision prism is required so that the pupil remains substantially centered on the optical axis. A dove prism is inexpensive since it requires only one (unsilvered) reflecting surface and the angles are conveniently but not necessarily 45, and do not have to be held to a very close tolerance. If the light is collimated as it traverses the prism, there are no aberrations introduced except some relatively unimportant one-sided aberrations of the pupil. Even if there is a small degree of convergence or divergence, the aberraions still are negligible. The prism can be made of flint glass, which is relatively inexpensive.

A roof prism can be used if preferred, but it is much more expensive, and the addition of the roof reflection is merely equivalent to turning the film over in the film plane.

This table gives the radii of curvature R of the optical surfaces, the axial thicknesses t of the optical element, the spaces s between elements, the refractive indices N and the conventional dispersive indices V, each numbered In the accompanying drawing: 5 by subscripts from front to rear. The last space, s

The single figure shows a zoom system according to is the distance to the film plane G. The unit of measurea preferred form of the invention. ment is one inch.

In the figure the zooming member 2 consists of a The prism is a conventional dove prism with 45 triplet component L5L8L'] and a singlet L and is axially angles and made of a high index glass, N=1.751. The aligned between a front compensating member 1 and a 10 dispersion is not critical, and I prefer flint glass because rear fixed member comprising a negative collimating it is relatively inexpensive. component 3, an image-rotation prism 4 and a prime The embodiments of the invention in which an exlens 5. The front compensating member 1 consists of clusive property or privilege is claimed are defined as a negative triplet component L L L and a biconcave follows: singlet L all in accordance with a preferred form of the A zoom lens system of the mechanical compensation invention. The prime lens 5 consists of a plurality of type for imaging an object area onto an image plane positive elements L L and L and a biconcave comat a fixed finite distance therefrom, comprising a movable ponent L between two of them in accordance with a positive zooming member in axial alignment with and preferred feature of the invention. between a fixed rear positive member and a movable The system is intended for the projection of a variable front negative compensating member, in which the zoomfilm area 6 onto a screen 7 of approximately constant mg member consists of a biconvex triplet component size, although obviously it can be used for blowing up in front of a single positive element convex to the front a central portion of a film area for viewing at a larger and the compensating member consists of a negative magnification. triplet concave toward the rear and in front of a single One of the triplet components is cemented and the biconcave element, and in which the power of the front other has small airspaces. These two structures are consurface of the biconcave element is greater than the sidered to be equivalent, in conformity with the usual power of the rear surface thereof and the power of the custom, rear surface of the positive element is greater than the The following table gives details of construction of one power of the front surface thereof, and in which detailed example according to a preferred form of the invention: data is as follows:

20.0 (Telephoto) 20.0 (Telephoto) Magnification= 28.3 (Median) Magn1fication= 28.3 (Median) 40.0 (Wide Angle) 40.0 (Wide Angle) Track Length =35.5 inches Track Length =35.5 inches R =+16."0 R =+l6.70 L n =.o7o N. =1.e2o v1 =eo.a t. =.o7o N, =1.s2o v. 60.3

R: =+1.516 R: =+1.516 L, t, =.650 N, =1.751 v, =27.s L:--- t, =.650 N, =1.751 V. =27.s

Ra =7.115 Ra =7.l15 L.-- t, =.07o N;=1.689 V;=30.9 40 t; =.o10 N;=1.689 V;=s0.9

R4 =+2.100 R4 =+2.1oo

ai =.220 i =-220 R =-2.s94 R6 =-2.s94 L J4 =.0ss N =1.734 V =51.2 4..- t4 =.088 N =1.734 V. =51.2

R4 =+2.4a4 R. =+2.4s4

.885 (Tele hoto) .885 (Telephoto) a; 1.912 (Med an) a; 1.912 (Median) 2.842 (Wide Angle) 2.842 (Wide Angle) R1 =+2 394 R1 =+2.9s4 L I t. =.a5o N =1.651 V =55.s Li..- n =.sso N; =1.e51 vi =55.s

Ra =2 434 s a: .005 s; =.005 R. =-2 660 R0 =-2.660 L; t. =.07o N. =1.751 V. =27.s e--- a =.o N. =1.751 V. =27.8

Rm=+1.912 io=+L 12 R +2434 3 =.040 50 R 434 at =.04o L e1 =.400 N1 =1.e51 v1 =55.s L1...{ :1 =.4oo N1 =1.e51 v1 =s5.s

R13=2.258 Rn=2.258

a; =.007 n =.007 R ;=+1.720 u=+1-720 LL" 1. 192 N5 =l.697 V9 =56.2 LB--- tr =.192 Na =l.697 Va =56.2

IMF-+1054 RH=+10-54 .7038 (Telephoto) a .7038 (Telephoto) as .2646 (Median) .2646 (Median) .0745 (Wide Angle) .0745 (Wide Angle) R|5=-2.434 l5=- 3 Lv...{ t, =.135 N. =1.751 V, =27.s L0"- R 775 a =.135 No =1.751 V0 =27.s

R =-.775 n=. L I ill =.035 io=1-6 io=56.2 40--- tio =.035 N =1.697 V1=56.2

R,,=+2.019 0 R11=+2.0i9

81 =.260 81 =.260 R15=Q i8= L1]... in =1.070 N11=L751 V =27.8 ll--- til =1-070 Nu=1.751 V11=27.8

B g=m li= B 1280 g. 326 R +1280 85 =.326

Ill L,,. 1,, =.22o N11=1.697 v,,=56.2 L1:---{ in =.22o N,,=1.e97 V=5e.2

Rn=2.883 65 Rn=2.883 R 1 8 a, =.l10 R 1818 o .81 a= L" a g 40() Nu=1.751 V13=27.8 iz---{ 21: 00 i:=1.751 Vi:= '8

Ru=+1.190 R13=+L190 B +4 8 0 =.115 R +4 001 810 =.115

.001 z|= L t =.230 N=1.620 vn=so.3 70 L...{ z =.23o N =1.620 V14=60.3

ll =-0 0 8n ==-010 B,.=+1.664 Rn=+1.6e4 L I N15=L620 V15=60.3 L15--- he =.190 N15=L620 v 5=60.3

Rg1=12.85 Rz1=12.85

BF=1.03 BF=1.03

wherein L is the element; R is the radius of curvature No references cited.

of the optical surface; t is the axial thickness of the optical element; s is the space between elements; N 15 the index R. L STERN, Assistant Examiner. of refraction; and V is the conventional dispersive index. 5

JEWELL H. PEDERSEN, Primary Examiner. 

